Reactors ultrasonic

Chand R, Ince NH, Gogate PR, Bremner DH (2009) Phenol degradation using 20, 300 and 520 kHz ultrasonic reactors with hydrogen peroxide, ozone and zero valent metals. Sep Purif Technol 67 103-109... 

The influence of pressure on US -assisted digestion has hardly been studied at all. There are only a few cases of chemical reaction acceleration where high pressure has been applied in closed ultrasonic reactors. These devices can also be used as ultrasonic digestors. 

Analyte oxidation and reagent generation in flow systems using tubular ultrasonic reactors have so far been unsuccessful [39], This is another under-explored area for analytical chemists. 

In another (somewhat different) approach, a probability density function (PDF) has been proposed (Moholkar and Pandit, 1997). This is used to map the cavity dynamics in the reaction medium covering all three phases of a cavity s lifetime growth, oscillation, and collapse. An ultrasonic reactor is considered highly efficient if the PDF shows peaks in the collapse regime at all of the locations in the cavitation field. This is an indication that pressure pulses exist throughout the medium and are not restricted to just a few locations. In other words, the cavitational intensity is uniformly distributed. If peaks occur in the growth and collapse regimes, it is desirable to place the reactor inside the sonicated medium at a location where the maximum probability of collapse is indicated. 

The development of the correlation is based on the global rates of reaction, i.e. considering the entire available volume for the reaction in the equipment, whereas Naidu et al. (1994) placed the test tube containing the reacting medium at a particular location where there was a maximum intensity of cavitation as detected by the hydrophones. The intensities of cavitation are different at different points in the ultrasonic reactor, which will result into different rates of reaction. The mapping studies... 

The first step in the progression of a sonochemical process from laboratory to large scale is to determine whether the ultrasonic enhancement is the result of a mechanical or a truly chemical effect. If it is mechanical then ultrasonic pre-treatment of a slurry may be all that is required before the reacting system is subjected to a subsequent conventional type reaction. If the effect is truly sonochemical, however, then sonication must be provided during the reaction itself. The second decision to be made is whether the reactor should be of the batch or flow type. Whichever type is to be used there are only three basic ways in which ultrasonic energy can be introduced to the reacting medium (Table 10.9). Several different types of ultrasonic reactors are currently available (Table 10.10). 

An ultrasonic reactor designed for sonochemical reactions is shown in the figure below. The main improvement is that the reactor is attached directly to the transducer. This set-up allows a direct transmission of the acoustic energy to the solution. Other advantages are the elimination of substrate contamination by probe erosion and a reproducible adjustment of a reaction vessel on the high energy spot in water baths. 

Polackova et alP studied the benzilic acid rearrangement of 4 under phase-transfer conditions with the effect of ultrasound. No rearrangement was observed when 50% KOH solution-toluene and benzyltriethylammo-nium chloride (TEBA) was used as phase transfer catalyst. The yields increased considerably to 60% when a powdered KOH was used instead of pellets. Ultrasonic reactor with the horn immersed into the reaction mixture proved to be much more effective, and 84% of the benzilic acid 5 was isolated after 15 min sonication even when KOH pellets were used. 

Figure 8.20 Schematic drawings of (a) barrel and (b) coaxial ultrasonic reactors [74],...

Figure 8.22 Schematic drawing of the ultrasonic reactor with a slit die with positions of horns [77].

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